Barre chords are a type of guitar chord where one finger presses down multiple strings across a single fret. This technique allows guitarists to play chords in different positions on the neck, making it easier to change keys without learning new fingerings. To form a barre chord, typically the index finger acts as a "bar," while the other fingers shape the chord. Common barre chord shapes include the E major and A major forms, which can be moved up and down the neck. Mastering barre chords is essential for guitarists looking to expand their playing versatility.